It has been known that cortical auditory evoked potentials (CAEP) elicited by speech sounds in humans generally exhibit characteristic waveshapes and scalp topography in response to specific acoustic features of the sounds, such as onset of voicing (VOT) and place of articulation. Electrophysiologic assessment of peripheral, brainstem and cortical responses to sound in young high-risk infants indicate that one or more levels of the auditory system may be involved in impaired auditory processing. Initial studies of language development in these infants also suggest that deviant cortical auditory processing in the young infant may be associated with poorer early language acquisition than infants with normal cortical responses to sound. So that possible remedial action may be taken as early as possible, early detection of impaired auditory processing is important.
To increase the usefulness of these electrophysiological measures, it is desirable to improve their specificity and sensitivity. To this end, for example, there is needed an improvement in techniques for the analysis of the pattern of auditory event related potentials (AERP) to speech and non-speech sounds.
It is known that the potentials recorded from the scalp derive from volume currents that originate in neuronal transmembrane currents within the brain. Active brain regions whose neurons are similarly oriented are capable of generating volume currents of sufficient magnitude to pass through the brain and its coverings, although markedly attenuated, where they are sensed as potential differences at the surface of the scalp. These volume currents must pass out of and back into the brain in order to complete the electrical circuit required by the conservation cf charge. A good deal of the current flow within the scalp, however, is parallel to this surface rather than transcranial. Thus, the potential differences associated with lateral currents do not directly reflect transcranial flow and serve to diffuse the recorded potential distributions at the scalp surface. It has been suggested that the Laplacian derivation, which is proportional to the second spatial derivative of the field potential, is a measure of the current flow perpendicular to the recording surface, and therefore, of transcranial current flow. This measure provides a substantially more focused estimate of the intracranial sources of electrical activity than does the field potential distribution. It is also essentially reference independent, an important consideration in interpreting topographic data.
In practice it turns out that computation of the Laplacian requires a reasonably accurate estimate of the actual potential surface. It has been attempted to derive the Laplacian directly from the recording electrodes by a differential operator implemented by analog circuits in which the Laplacian at a particular point is derived as the average of the gradients from a number of surrounding recorded points. However, these direct analog attempts have not proven particularly successful in mapping an area because of the limited number of points available for use in the computation. In these attempts, the points used have been only the points directly measured and practical reasons limit the number of points that can be directly measured. In particular, the attempt to do such a Laplacian derivation directly with fifteen or twenty points, the limit generally thought practical to measure directly conveniently, has not proved satisfactory.
Similarly, in other electrophysiological processes involving the analysis of the pattern of neural potentials in both humans and animals, there can be used to advantage better techniques for mapping and/or displaying such potentials.